Near Vertical Incidence Skywave (NVIS)And The 40 meter
Novice Sub-band. By:Carl W. Davis W8WZ
You may
have seen this article on eham.net. Carl has kindly allowed us to
repeat it here slightly edited to share with more Hams!*******

Forty meters has always held a special attraction for
me. The personality of 40 lends itself to casual rag chewing contacts
across the Unites States and Canada. The novice sub-band was my first real
introduction to HF since I got my ticket when ten meters was totally dead
and there was not enough room in my parent's yard for an 80 meter dipole.
My first 40 meter antenna was a flat-top dipole center fed with 300
ohm television twin lead and up about 45 feet.I have been operating on
40 meters ever since. Today, I am fortunate enough to live deep in
Indiana farm country where I have freedom to experiment with antenna
design to my heart's content. For 40 meters, I have an OFC Dipole very
similar to the Fritzel FD4 in an ?Inverted V? formation with the apex at
75 feet. I also have a gap Titan DX vertical. And, until a short time ago,
I had a half wave 40 meter dipole sloping south with the feed point about
40 feet above ground. As I used those antennas, I realized that very
rarely (if ever) did my sloping dipole ever outperform my Fritzel. So, I
decided to take down the sloping dipole and use the wire and coax to make
something new. But what???? I also had half a roll of 14 gage stranded
insulated wire in the shed that could be used for this project. My
first thought was a phased array for 40. However, after looking into that,
I realized that I would have to move some of my other antennas for that
idea to work. Being lazy, and also not wanting to change the performance
of my other antennas, I decided to do something less invasive. As I looked
for ideas I came across a few web pages for NVIS. Near Vertical
Incidence Skywave involves placing a horizontally polarized antenna very
close to the ground. The result is that your RF power is radiated largely
straight up into the sky at a VERY HIGH angle of radiation. Also, there is
limited ground wave propagation of your transmitted signal. Basically, you
create a large ?ball? of RF with the bulk of it going straight up into the
sky. How in the world could that be useful to a radio amateur? I wondered.
The answer is that this type of antenna can fill a void that our more
traditional antenna systems leave out. While NVIS systems will not work
well for DX - the bubble of RF that they create results in a very strong
RF signal to all stations within that bubble. The size of that RF ball or
bubble is determined by the selected transmit frequency and to a lesser
extent propagation conditions. 40 meters is the highest ham band that NVIS
works well on. An NVIS system creates a strong uniform signal over a small
geographic area. In short: only close stations can hear you - but they
will hear you well. Since I had a very good DX 40 meter antenna, I
decided to try out the NVIS concept.

THE
PROJECT:

The NVIS system I
chose is very simple. It is described in an article by K5AXN which can be
found on the internet at http://www.hamuniverse.com/supernvis.html.Basically, It consists of a 40 meter dipole hung in a flat-top
configuration at an elevation of about seven feet. Underneath it is a
system of 3 RF reflectors. The RF reflectors are 60 foot sections of #14
stranded insulated wire. One of these reflectors was laid on the ground
directly under the dipole. The other two were laid on the ground parallel
to the dipole. One was laid 6 feet South of the Dipole, the other was laid
6 feet North of it. (See graphic in the Hamuniverse.com link above.)
The antenna's need to be placed so low presented two problems for me.
First, this design presented a safety concern. We all know that good
amateur practice is to keep the antenna high enough so that it cannot
cause injury to a passer by who may inadvertently touch it. Secondly, an
antenna that low would be in the way when I did yard work such as mowing
the lawn. Finally, how was I going to place the RF reflectors on the
ground and still be able to mow the grass? I have never liked buried
radials because they work themselves up over time and get tangled in lawn
mower blades. So, I didn't even consider the idea of burying
them.Instead I looked around my property for a place that this antenna
system could work. Directly South of my house is a small but thick
woods. I decided to put my NVIS system in the woods. NVIS has been used by
military radio operators for years. They have used this technique in dense
forests, so I was confident that the idea would work. Although I did
figure I would need to do much antenna trimming to compensate for its
interaction with all the trees. This would be the safest location possible
since no visitors to my house would accidentally go into the thick brush.
Also, since I didn't need to mow the grass in the woods, the wire
reflectors could lay flat on the earth, held in place with plastic tent
stakes. I tied the dipole to trees at 7 feet and supported the antenna
at a point in the middle of each leg to prevent sag. To support them at
those places, I cut one foot sections of ? inch PVC pipe. I drilled holes
in each end. I threaded the antenna wire through the hole in one end of
the PVC pipe. Then I hung the PVC pipe from tree branches by putting nylon
twine through the end of the PVC pipe opposite the antenna, and tied it to
a tree branch. I supported the center of the dipole by hanging it from a
tree branch in similar fashion. I had purchased some ?? PVC pipe to use as
a center support mast but found that I did not need it.

SURPRISE!To my surprise, I
found the antenna to be resonant on the same frequencies as it was when
hanging as a sloping dipole in free space. In this case that was the lower
end of 40 with the lowest SWR between 7.000-7.040 MC. I made some
contacts with the antenna this way. I found that it was less prone to BCI
and atmospheric noise than my other antennas. It was very quiet. Amateur
signals were also quieter than on other antennas. When I found that I had
a much lower RST from other stations, I figured that the antenna was quiet
because it was nothing but an air-cooled dummy load. At this point I was
disappointed with the antenna, and figured that its ability to limit the
BCI and QRN was the same as my attenuator's ability to do the same - but
at least the attenuator didn't effect my transmit signal. But, I knew I
was not finished with the project. I still needed to put down the RF
reflectors. I just wanted to see how much difference they made, so I made
some contacts without them.

THE ANTENNA SYSTEM
COMES ALIVE!Once I added the RF reflectors - the antenna
system came alive! As anticipated they did have an effect on the resonant
frequency of the dipole. They moved the resonant frequency right into the
Novice Sub-band. Exactly where the BCI is worst and where I really wanted
to use this antenna! I was very happy that I wouldn't need to do the
antenna pruning/adjusting that I thought I would.

SOME GOOD POINTS ABOUT BCI AND QRN REJECTION USING
NVIS ANTENNAS! Here is what I found
using this antenna in the Novice Sub-band. First - it is a quiet antenna.
Of all my antennas, it rejects the most BCI and QRN. The Broadcast signals
are usually 10 - 15 S-units lower on the NVIS system. The Amateur signals
are generally about 1-3 S-units lower on the NVIS system. Sometimes, the
amateur signals have the same signal strength. Very rarely, are they
louder. However, even though the amateur signals are usually lower in
strength, they are easier to copy because the BCI has been drastically
reduced. A typical contact goes like this: On normal antenna: Amateur
Signal is S-7 and noise is S-6. That means that the signal I want to copy
is only one S-unit above the noise. On the NVIS system the amateur signal
is only an S5. However, the noise is only an S1 or S2. Thus, the signal I
want to copy is now 3 to 4 S-units louder than the noise .

STATESIDE DX?I have routinely gotten very good
signal reports from the stations I work. The best DX I got with this
antenna system so far was a station in Georgia, USA. He gave me 599 and I
sent him 599. It is definitely not a DX antenna. However, it does allow me
to have many loud contacts with stations within about a 500 mile radius of
me. By rejecting the BCI so well, it also allows me to use frequencies
that I would have otherwise just tuned past due to the QRM.

SOME FINAL POINTS TO PONDER ABOUT NVIS ANTENNAS!Do I suggest NVIS for others? If you already have a ?normal? 40
meter antenna and want to expand - this is a great way to do it. If you
are looking for an easy antenna project that doesn't involve a lot of tree
climbing or tower raising - this is a great one. If you want an
antenna system you can place in the woods and free up your open space for
other antennas, this is one of the few antennas that will do that well.
If you are a new ham limited to the Novice Sub-bands and just want a
simple, inexpensive antenna system that will reject as much BCI as
possible and let you practice your CW skills by making loud and clear
contacts with hams within 500 miles of you - this is a good antenna for
you. Also, if you need a 40 meter antenna to check into local or state
wide nets, the NVIS system would be good for you too. You wouldn't be
causing much QRM to stations outside of your net's service area. And, your
signal would be uniformly loud to all of the stations within your net's
service area. For that sort of application - this is a good antenna as
well. ************Disclaimer: This article isn't
designed to be a technical discussion of NVIS. It is intended only to be a
description of a practical application of NVIS on the 40 meter Novice
Sub-band. For a better description of how NVIS works, the author suggests
that readers do their own technical research and personally experiment
with NVIS systems.73 Carl, W8WZQuestions?
Contact Carlatwww.QRZ.COMfor his email address.Super NVIS
project referenced in this article here